A lenticular image fixed in a recording media, such as a photographic print or transparency media or displayed on a CRT display, is termed a lenticular print and comprises "bundles" of image lines interleaved from a plurality of images of a scene taken from different points of view. Each bundle contains an image line from each of the original images in sequence, and all image bundles are equal in width. When image bundles of equal width abut one another, the width of a bundle is its "pitch." When image bundles of equal width are spaced apart equally, the pitch is the sum of the bundle width and width of the space between bundles. The image lines are created typically by scanning lines of a digital printer or display. A lenticular overlay, or face plate, comprising a plurality of oriented lenticules having the same pitch as the bundles in the lenticular print, when placed over the media and in proper alignment with the image lines, projects the plurality of images at different viewing angles corresponding to the viewing angles of the original scene, and provides an image which evokes a sense of depth to a human viewer.
Generally, when depth images are viewed, the orientation of the lenticules is vertical, i.e., the axes of the cylindrical lens segments run up and down relative to the viewer, although some applications require horizontal lenticules. The invention is equally useful to either orientation. In vertical uses, as the viewer moves his/her head in a lateral direction, new and different perspective views are seen by each of the viewer's eyes, thereby creating not only an autostereoscopic depth image but some "look-around" effect as well. Each perspective view is a result of a set of exposed image or scan lines, one from each bundle under each lenticule, the composite of which comprises the entire view. As the eye is moved laterally, a new set of scanned lines becomes visible as a new composite which in turn comprises the new perspective view.
Image changes are preferably small and gradual as one might expect when viewing a real scene and slowly moving one's head in a lateral direction. However, when the viewer moves past the last image lines in the bundles directly under the lenticules, the first image lines in the adjacent bundles become visible which constitutes an abrupt image shift back to the initial perspective. This effect is termed "image break" and represents a point in the angular space in front of a lenticular print where the primary image is no longer visible, and a "satellite" image appears. Satellite images are equally valid representations of the object scene and serve to increase the total range of angular space that multiple viewers can simultaneously enjoy in a given lenticular picture. However, it is important that the primary image be properly centered to the lenticular array (translational alignment) and that the scan lines and the lenticular axes be parallel to each other (rotational alignment). If the primary image is not centered, its projection space will not be centered on a normal to the print surface which leads to viewer confusion as to the proper viewing angle for the print. If the scan lines are not parallel to the lenticule axes, the image break is seen as an angled moire pattern superimposed over the area image which detracts from the viewer's enjoyment of the display.
In some applications, depth imaging of a single scene is not the objective, but rather two or more completely different depth or even non-depth images are visible depending upon the angle of view, such as, for example, text in one view and pictorial content in a second view. A different type of image break from one image to the next within the bundles is essential to the success of such lenticular applications, and highly-accurate alignment of overlay with print is essential to uniform, sharp image break.
Various approaches have been proposed to ensure that a lenticular image print and a lenticular overlay will be properly aligned during their assembly. The simplest approach typically is for a human operator, using transmitted or reflected illumination, to manipulate either the lenticular print or the overlay or both until all moire lines are eliminated and the depth effect "looks right." This approach can yield excellent image prints, but it is time-consuming, labor-intensive, and not amenable to mechanization and mass production. Optimum translational alignment is also difficult to achieve. This approach also depends upon there being "recognizable" subject matter in the print by which an operator can judge the results of his actions. Unconventional images such as abstract designs, and unconventional uses for depth imaging such as three-dimensional radiography, may provide an operator with few inherent clues as to proper alignment.
Another approach has been to include alignment lines on the margin of the print outside the image area, the lines having a fixed and known lateral relationship to the image bundles, and to scan these alignment lines for alignment with the centers of the overlying lenticules either visually or by any well-known computer image recognition system while moving the overlay or print, first rotationally to eliminate moire lines which is indicated by achieving uniform brightness along the alignment lines, and then translationally to maximize brightness along the alignment lines when the lenticules are centered over the alignment lines. No reference is made or required to the actual lenticular image area for alignment.
A serious drawback of such approaches is that they align directly the centers of the lenticules in the overlay with the centers of the image bundles in the lenticular print by using a visual or electronic signal generated from the central region of the lenticules. However, the slope of the brightness curve is very shallow in the vicinity of maximum brightness at the center of a lenticule, which means that there is a "dead zone" near the center of a lenticule in which relatively large translational movements of the overlay causes relatively small changes in brightness. In this zone, alignment accuracy is low and probability is high that the lenticules of the overlay will not be optimally disposed, that is, accurately centered over the bundles of the lenticular print.
It is a primary object of the subject invention to provide a method and apparatus whereby a lenticular print and a lenticular overlay can be accurately aligned, both rotationally and translationally, by a human operator.
It is a further object of the invention to provide a method and apparatus whereby a lenticular print and a lenticular overlay can be accurately aligned automatically.
It is a further object of the invention to provide two-dimensional fiducial indicia having one or more alignment elements on a lenticular print whereby a lenticular overlay can be accurately aligned with the lenticular print.